Associated with further progress of miniaturization, weight saving and multifunctionality of electronic devices in recent years, integration of LSI and other chip devices proceeds, and the configurations thereof are rapidly changed to ones with an increased number of pins and a reduced size. Accordingly, a multilayer printed wiring board therefor is being developed to have a fine wiring structure for enhancing the mounting density of electronic parts.
As a production method of a multilayer printed wiring board satisfying the demands, a build-up method has been known and is becoming a mainstream of the technique suitable for weight saving, miniaturization and fine structure.
There is an active movement of restricting a material that has possibility of generating a harmful substance upon combustion including electronic parts, according to increase of the environmental conservation awareness. An ordinary multilayer printed wiring board uses a bromine compound for achieving flame retardancy but has possibility of generating a harmful substance upon combustion, and therefore it is expected that the bromine compound may not be used in the near future.
Lead-free solder containing no lead is being practically used as solder that is generally used for connecting the electronic parts to a multilayer printed wiring board. The lead-free solder is used at a temperature that is higher by approximately from 20 to 30° C. than that for ordinary eutectic solder, and therefore the materials necessarily have higher heat resistance.
In the multilayer printed wiring board having the build-up structure, via holes are being filled or having a stack, associated with the increase of the number of layers, for enhancing the density. For decreasing the thickness of the multilayer printed wiring board, however, an insulating resin layer containing no glass cloth has a tendency of having an increased thermal expansion coefficient, and the difference in thermal expansion coefficient from copper in a via hole being filled or having a stack largely affects the reliability of connection, which may be fears about reliability. Under the circumstances, a material that has a small thermal expansion coefficient is being demanded for the insulating resin layer.
For decreasing the thermal expansion coefficient of the insulating resin layer, such a method has been employed that an inorganic filler having a small thermal expansion coefficient is filled in a large amount, thereby decreasing the thermal expansion coefficient of the entire insulating layer (see, for example, Patent Document 1). However, the method may cause various problems including decrease of the fluidity and decrease of the insulation reliability.
There are attempts for achieving low thermal expansion by selecting or improving the resins. For example, there is a resin composition for press molding having low thermal expansion property using an epoxy resin having a bifunctional naphthalene skeleton or biphenyl skeleton as an example of an epoxy resin having an aromatic ring (see Patent Document 2), in which the filler is filled in an amount of from 80 to 92.5% by volume. Furthermore, decrease of thermal expansion property of a resin composition for a wiring board has been generally achieved by increasing the crosslinking density and increasing the glass transition temperature (Tg) (see Patent Documents 3 and 4). However, the increase of the crosslinking density requires shortening the molecular chain between the functional groups, and it is difficult to shorten the molecular chain beyond a certain length from the standpoint of the reactivity, the strength of the resin, and the like.
There is an attempt of introducing an imide skeleton, which is considered to be effective for achieving heat resistance and low thermal expansion, and for example, a thermosetting composition for build-up using an aromatic diamine having an imide group and an epoxy resin has been proposed (see Patent Document 5). However, in the case where a low molecular weight polyimide compound is used as a curing agent of the epoxy resin, the characteristics of the composition may have substantially no difference from those of the epoxy resin in many cases.
A laminate plate for a printed wiring board is ordinarily a laminate plate formed by curing and integratedly molding a resin composition containing an epoxy resin as a major ingredient with a glass woven cloth. The epoxy resin is excellent in balance among the insulating property, the heat resistance, the cost and the like, but has a limitation in enhancing the heat resistance for addressing the demand of increased heat resistance associated with the high density mounting and the highly multilayered structure of the printed wiring board in recent years.
Patent Document 2 described above achieves low thermal expansion by selecting an epoxy resin having an aromatic ring and filling an inorganic filler, such as silica, to a high density, since an epoxy resin has a large thermal expansion coefficient. However, it has been known that the increase of the filled amount of the inorganic filler results in decrease of the insulation reliability, insufficient adhesion between the resin and the wiring layer, and failure on press molding due to moisture absorption.
A polybismaleimide resin, which has been widely used in a high density mounting and highly multilayered laminate plate, is excellent in heat resistance, but has high hygroscopicity and a problem on adhesion. Furthermore, as compared to an epoxy resin, the polybismaleimide resin has a defect of low productivity since a high temperature and a prolonged period of time are required upon lamination. Specifically, an ordinary epoxy resin may be cured at a temperature of 180° C. or lower, but lamination of the polybismaleimide resin requires a process with a high temperature of 220° C. or higher for a prolonged period of time.
Accordingly, there have been proposals of using a modified imide resin containing a polybismaleimide resin modified with an epoxy resin having a naphthalene skeleton (see, for example, Patent Document 6). The modified imide resin is improved in hygroscopicity and adhesion property. However, the modified imide resin is obtained by modifying with a low molecular weight compound containing a hydroxyl group and an epoxy group for imparting solubility to a common solvent, such as methyl ethyl ketone, and thus the resulting modified imide resin is largely inferior in heat resistance as compared to the polybismaleimide resin.
A varnish and a prepreg of a resin composition for a printed wiring board are required to have storage stability, and thus necessarily use such a material that has high potential reactivity to a curing agent and a curing accelerator (reaction potential), but achieves long term storage of the resin composition. In the phenol curing system and the aromatic amine curing system, an imidazole compound may be favorably used as a curing accelerator, but these are insufficient in reaction potential and are difficult to be stored for a prolonged period of time.